1. Field of the Invention
The present invention relates to a method of and an apparatus for covering permanent magnets of a motor rotor with a cylindrical sleeve.
2. Description of the Related Art
Electric motors generally comprise a stator core having as many windings as the number of phases and a rotor rotatably disposed in the stator core and having a plurality of permanent magnets disposed on its outer circumferential surface. The rotor has a shaft and a yoke with the permanent magnets positioned at angularly spaced intervals on the outer circumferential surface of the yoke. The permanent magnets are covered with a cylindrical sleeve made of fiber-reinforced synthetic resin and held against the outer circumferential surface of the yoke.
In order to keep the permanent magnets firmly against the outer circumferential surface of the yoke, it is necessary that the cylindrical sleeve be securely held in intimate contact with the permanent magnets. To meet such a requirement, the diameter of the cylindrical member is considerably smaller than the diameter of the rotor. Therefore, it is a tedious and time-consuming task to install the cylindrical member on the yoke to cover the permanent magnets.
Japanese laid-open patent publication No. 55-111645 discloses an automatic magnet inserting device which comprises means for transferring an array of unmagnetized magnets to a magnetizing position, means for magnetizing the transferred magnets to a predetermined magnetized state in the magnetizing position, means for converting the shape of the magnetized magnets to a cylindrical shape, and means for pushing the cylindrical assembly of the magnets in an axial direction thereof and simultaneously inserting the magnets into a rotor case.
According to the disclosed automatic magnet inserting device, the magnets are simultaneously inserted into the rotor case with a jig and secured in position by an adhesive. To hold the magnets intimately against the outer circumferential surface of the yoke, the yoke has to be pressed into the magnets that have been fixedly positioned in the rotor case. Such a process of pressing the yoke into the magnets is cumbersome. The cost of rotors manufactured by the automatic magnet inserting device cannot be reduced because of the use of the adhesive for securing the magnets.
According to Japanese laid-open patent publication No. 7-336966, there is known a process of covering permanent magnets with a cylindrical member by forcibly spreading the cylindrical member radially outwardly under a fluid pressure, inserting a rotor and the permanent magnets into the cylindrical member, and then discharging the fluid.
The disclosed process will be described below with reference to FIGS. 33A and 33B of the accompanying drawings. As shown in FIG. 33A, a rotor 2 and permanent magnets 3 are supported by a holder jig 4 and a movable jig 5. A cylindrical sleeve 6 is disposed in a casing 7 and has an end spread and gripped by an O-ring 8 mounted in the casing 7, the holder jig 4, and the movable jig 5.
A fluid is introduced into the casing 7 to spread the cylindrical sleeve 6 radially outwardly under a fluid pressure. Then, the rotor 2 and the permanent magnets 3 are inserted together with the movable jig 5 into the cylindrical sleeve 6.
When the movable jig 5 is inserted into the cylindrical sleeve 6, the cylindrical sleeve 6 is contracted radially inwardly between the movable jig 5 and the holder jig 4, and pulled to a straight shape between teeth 4a of the holder jig 4, as show in FIG. 33B. The permanent magnets 3 inserted together with the movable jig 5 into the cylindrical sleeve 6 are held in sliding contact with the straight portions of the cylindrical sleeve 6. Therefore, surfaces of the permanent magnets 3 and the cylindrical sleeve 6 are liable to be damaged due to the frictional engagement therebetween.
Cylindrical members often have mechanical strength variations. A cylindrical member which tends to stretch under a given pressure is unable to impose tightening forces that are required to prevent permanent magnets from being displaced off the rotor. On the other hand, a cylindrical member which is less likely to stretch under a given pressure exerts resistance to the insertion of permanent magnets into the cylindrical member. Even if the permanent magnets can be covered with the cylindrical member, the cylindrical member will be cracked by centrifugal forces developed when the rotor rotates upon energization of the motor.